Programming and Dynamic Control of the Circular Polarization of Luminescence from an Achiral Fluorescent Dye in a Liquid Crystal Host by Molecular Motors

Abstract Circular polarized light is utilized in communication and display technologies and a major challenge is to develop systems that can be switched between left and right circular polarized luminescence with high degrees of polarization and enable multiple addressable stable states. Luminescent dyes in Liquid Crystal (LC) cholesteric phases are attractive systems to generate, amplify and modulate circularly polarized luminescence (CPL). In the present study, we employ light‐driven molecular motors as photo‐controlled chiral dopants in LCs to switch the handedness of the LC and the circular polarization of luminescence from an achiral dye embedded in the mesogenic material. Tuning of the color of the CPL and the retention time of the photoprogrammed helicity is demonstrated making use of a variety of motors and dyes. The flexibility offered by the design based on inherently chiral unidirectional rotary motors provides full control over CPL non‐invasively by light, opening possibilities for pixilated displays with externally addressable polarization.

from 3 to 25 μm. UV/vis absorption spectra were obtained with an Agilent 8453 UV-vis Diode Array System at room temperature. Circular dichroism (CD) spectra were recorded on a JASCO J-715 or a JASCO 810 spectropolarimeter at room temperature. Results are plotted as circular differential absorbance AL-AR, i.e. the difference between the absorbances for left and right-circularly polarized light. The circular differential absorbance, a dimensionless quantity also abbreviated as A, is proportional to the ellipticity  in millidegrees via: A =  [mdeg] . [9] Fluorescence spectra were collected with a JASCO FP-6200 spectrometer. Generalized ellipsometry was performed on a VASE instrument from Woollam. Irradiation experiments were performed using either LED lamps (365 nm, 405 nm, 455 nm, 470 nm, 530 nm and 617 nm) obtained from Thorlabs Inc or a handhold LED lamp (312 nm). All optical phenomena of the LC mixtures were observed and recorded via a polarized optical microscope (POM, DM2700p, Leica).

CPL measurement
Circular Polarization of Luminescence measurements were performed using a home-built spectrometer, involving photoelastic modulation at 50 kHz, parallel multichannel detection and single photon counting electronics. [10] The instrument is capable of measuring both circular and linear polarizations.
In the polarization measurement, the emission collection and photoexcitation were performed in a direction perpendicular to the luminescent layer and with an in-line geometry. Excitation light was depolarized by sending it through an optical fiber. Figure S1. A Schematic representation of experimental setup for CPL measurement of film samples.

Synthesis of RB:
To a DMF solution (15 mL) of B (200 mg, 0.55 μmol), benzaldehyde (0.53 mL, 5.1 μmol), acetic acid (1 mL) and piperidine (1 mL) were added and the mixture was heated at reflux overnight. After cooling to room temperature, the mixture was diluted with water and extracted with ethyl acetate. The organic layer was washed with brine and dried over MgSO4. The solvent was evaporated under reduced pressure and the crude product was purified by column chromatography (SiO2, DCM:pentane = 1:4, v/v).

Sample preparation
BODIPY (BB, GB, OB or RB), molecular machine (M1, M2, M3 or M4) and LC mixture (E7 or ZLI-1132) were dissolved together in DCM and stirred at 60 o C until DCM was removed. The resulted mixture was filled into LC cells at 60 o C by capillary suction. Samples were allowed to cool down to room temperature after filling. Table S1. Compositions of MCELDs.

HTP measurement
Helical twisting power (HTP) of molecular machines and the changes in HTP upon light irradiation were determined by the Grandjean-Cano method. [11] HTP (ꞵ) is defined as follows: ꞵ = 1 / (pc), where p is the helical pitch and c is the molar or mass concentration. The pitch is determined by: p = 2R tanθ, where R represents the distance between the disclination lines and θ is the wedge angle of the wedge cells (tan θ =0.00785). A liquid crystal mixture was prepared by doping an enantiopure molecular machine into LC, and then it was filled into the wedge cell by capillary force. The wedge cell was heated to 60 ℃ then cooled down to room temperature with a cooling rate of -1 ℃·min -1 . The disclination lines were observed through POM, which is a characteristic of cholesteric organization of liquid crystal molecules. The length of R was determined by measuring the intervals between the disclination lines.

Photo luminescent quantum yield (PLQY) measurement
Background correction was done by subtracting the dark spectra from the corresponding reference or sample measurement spectrum. Then QY calculation was done according to the two-measurement approach as described by Leyre et al. [12]

Effect of circular dichroic self-absorption on the circular polarization of luminescence.
For organic fluorescent dyes there is almost invariable always some overlap between the fluorescence (S1→ S0) and the lowest absorption band (S1 S0). This implies that some of the photons emitted as fluorescence will be reabsorbed by (other) molecules in the solution or matrix. For solutions of chiral molecules, the probabilities for absorption of left and right circular polarized photons are not the same, and this circular selective self-absorption may contribute in an artificial manner to the circular polarization of luminescence.
To estimate the effect of circular dichroic self-absorption on the apparent degree of circular polarization of luminescence glum , we start from the definition of the dissymmetry factor Here it is implicitly assumed that the luminescence is generated in a thin layer at the side of the film facing the excitation source. Adopting this worst case scenario, we then estimate the artificial degree of circular polarization due to Circular Dichroic Self-Absorption (glum CDSA ) as :